Allow installing migrations into existing schema

Mostly for improved security. One postgres user could be used for creating the schema (and managing database objects, including `pgcrypto`), while Graphile worker runs with a different user with the appropriate privileges within it's schema. Also might allow for separation of concerns if, for instance, database objects are managed by a separate team.

This was previously requested in https://github.com/graphile/worker/issues/12, but I'm opening a new issue since the schema-name is already configurable, making this more feasible.